The present invention relates to coated optical elements and to methods for forming said coating.
In the following description and in the appended claims, the term xe2x80x9clens elementxe2x80x9d is used to indicate any coated or uncoated, at least partially transparent element capable of allowing vision through the same, such as for example powered and unpowered sun lenses and ophthalmic lenses for spectacles, visors, slabs, protective shields, lens blanks and wafers for laminating with the foregoing. Typically, such lens elements include a substrate made of homogeneous glass or plastic, in particular of polycarbonate or diethylenglycolbis-allyl-carbonate, commercially available as CR 39(trademark) (PPG Industries Inc.), both commonly used for manufacturing ophthalmic and non-ophthalmic lenses for glasses.
As is known, in the field of optical elements and in particular of lenses for spectacles it is useful to provide mirror coatings exhibiting, at the same time, a suitable adhesion, a good resistance to abrasion, a good reflectance, a low absorbance and, in some cases, a degree of coloration. According to the teaching of the prior art, desired characteristics of coloring and mirroring of the lens element are obtained by forming a coating including at least two layers of suitable materials on a transparent substrate of the lens element.
One type of conventional mirror coating consisting of two layers onlyxe2x80x94each layer has one or more specific functions. The first layer, generally consisting of a pure metal layer (e.g. chromium or aluminum) which functions both for adhering the coating directly to the substrate and for partially reflecting and partially absorbing light in the visible spectrum (400-700 nm). The second layer, generally consisting of silicon oxide, functions to impart a coloration to the lens and to impart a certain resistance to abrasion to the coating. The technologies used for manufacturing these low-cost two-layered coatings are those of physical vapor deposition, or PVD, based on a thermal evaporation mechanism wherein the heating of the material to be evaporated is substantially carried out by Joule effect. Although two-layered mirror coatings of the known type provide coloring and mirroring at a reasonable cost, nonetheless they possess the drawback, among others, of having properties of adhesion to the substrate and of resistance to abrasion which are not entirely satisfactory.
It is known in the art to provide a multi layer coating comprising a plurality of dielectric layers and dielectric interfaces. See, e.g., U.S. Pat. No. 3,679,291 to Apfel et al. Such xe2x80x9cdielectric stacksxe2x80x9d may be formed of alternating layers of high and low refractive index designed to provide desired optical properties such as the suppression of reflection. See, e.g., U.S. Pat. No. 5,719,705 to Machol assigned to Sola International, Inc. which discloses the use of alternating layers of titanium oxide and silicon dioxide deposited on a thin adhesion layer of chromium oxide.
Depending on the number and type of materials and the number and thickness of layers employed in conventional dielectric stacks, the coating may become relatively expensive and difficult to fabricate.
Accordingly, it is an object of the present invention to provide a readily and inexpensively formed coating for an optical element which is durable and can be tailored to produce desired reflectance properties.
It is another object of the present invention to provide a coating for an optical element having improved characteristics of adhesion and resistance to abrasion particularly with respect to conventional metal-dielectric two-layered coatings of the prior art.
According to the present invention, it has been found that the desired improvement of the characteristics of adhesion and of resistance to abrasion of the coating may be achieved by realizing the coating with a new combination of layers of suitable materials.
Thus, according to a first aspect thereof, the present invention provides coating for an optical element comprising a thin adhesion layer deposited on a surface of the lens element, a first dielectric layer having a refractive index substantially greater than the refraction index of the lens element at the surface and a second dielectric layer including a material having a refraction index substantially lower than the refraction index of the material of the first dielectric layer. The material of the first dielectric layer is selected from the group of CrxOy, TiO2, ZnSe ZnS and mixtures thereof wherein x is a number between 1 and 2 and y is a number between 1 and 3.
In this embodiment, the coating of the invention comprises alone, or in combination with other layers applied on the substrate, a sequence of at least three superimposed layers. A first relatively thin layer is intended for carrying out the function of adhesion to the substrate (or other intermediate coatings on the substrate) and does not appreciably affect the optical properties of the coated optical element. A second, dielectric layer forms two interfaces with adjoining material of different dielectric constants which reflect light. A third dielectric layer of a third material, produces interference effects and influences coloring of the optical element and, optionally, provides the desired resistance to abrasion.
Advantageously, thanks to said combination of layers, it is also possible to obtain a wider range of colors and a greater variety of shades with respect to those obtainable with the conventional two-layered coatings. The mirror-coated substrate of the invention is also easier to clean than the two-layered coatings of known type. Said advantageous feature may be attributed to the lower superficial roughness of the coating of the invention. Advantageously, due to its essential lack of absorption, the coating of the present invention exhibits a greater transmittance with respect to that of two-layered coatings, which permits the coating of dark substrates in a wider and more flexible way with respect to the prior art, while satisfying minimum transmittance requirements. This advantage is particularly appreciated in the case of lenses for sunglasses, since it is possible to widen the range of colors of the lens complying, at the same time, with the minimum transmittance limit of the coated lens, which limit is provided in the case of lenses to be worn when driving motor vehicles, and is fixed at 8% by the standards EN 1836, ANSI Z80.3 and As 1067.1.
Advantageously, the lower stiffness of the coating achieves better properties of adhesion, with reference to those possessed by the two-layered coatings of comparable cost.
In a preferred embodiment, and as it will better appear hereinafter, the optical element of the invention may be manufactured at a reasonable cost using conventional equipment and technology of vapor deposition. Preferably, the first thin layer has a thickness comprised between 0.5 and 5 nm and most preferably between 0.5 and 2 nm. For the purposes of the invention, the first material is preferably selected from the group comprising: Cr, Ti, SiO, SiO2, IN2O3, SnO2 and mixtures thereof. Among them, chromium is particularly preferred. Preferably, the first dielectric layer has a thickness comprised between 3 and 80 nm and most preferably between 8 and 50 nm. For the purposes of the invention, the material of the first dielectric layer is preferably selected from the group comprising: CrxOy, TiO2, ZnSe, ZnS and mixtures thereof, wherein x is a number comprised between 1 and 2, and y is a number comprised between 1 and 3. Among them, at least partially oxidized chromium is preferred. According to a preferred embodiment, the material of the first dielectric layer has a refraction index between 1.7 and 2.7. More preferably, the refraction index of this material is comprised between 1.9 and 2.4. In the present description, the refraction indexes of the different materials are intended as indexes measured at the wavelengths at which the respective materials are transparent or only weakly absorbent. Preferably, the second dielectric layer has a thickness between 5 and 400 nm and most preferably between 10 and 200 nm. For the purposes of the invention, the third material is preferably selected from the group comprising: SiO, SiO2, MgF2, Na5Al3,F14, and mixtures thereof. Preferably, the material of the second dielectric layer has a refraction index between 1.32 and 1.8 and more preferably between 1.4 and 1.7.
Preferably, the substrate essentially consists of a homogeneous transparent element of glass or plastic having a refraction index at 500 nm between 1.38 and 1.75. If the substrate is made of plastic, additional coatings may be applied to the substrate. Advantageously the coatings are a primer and a polysiloxane resin (hard coat) or other suitable material having the double function of imparting scratch-proof properties to the finished product, and of increasing the adhesion of the mirror coating. In this case the hard coat acts as a dielectric material which forms a dielectric interface with the first dielectric layer described above. For the purposes of the invention, the plastic substrate is preferably made of a material selected from the group comprising: acrylic-based polymers and copolymers, polymethylmethacrylate, polycarbonate, polyol-allyl-carbonates, cellulose esters, polyacrylates, polystyrene and polyurethanes. Among them, it is preferred to use an acrylic-based copolymer commercially available as Spectralite(trademark) (Sola Optical), diethylenglycolbis-allyl-carbonate, commercially available as CR 39(trademark) (PPG Industries Inc.), or polycarbonate, commonly used for manufacturing ophthalmic and non-ophthalmic lenses for glasses.
Preferably, the optical element is manufactured in the shape of lens or visor for glasses, or in the shape of a slab or protective shield. Where an edged lens is desired, the optical element may be a molded lens blank coated as described herein.
According to a second aspect thereof, the present invention provides a method for forming the coating described herein. Such a process may include the steps of placing an optical element such as a spectacle lens blank in a sealed chamber. The chamber is evacuated to a predetermined degree. The surface of the optical element is activated by means of ion discharge in an oxygen-containing environment for a predetermined period of time. A first, relatively, thin adhesion layer is deposited in substantial absence of oxygen. The first dielectric layer is then deposited in the presence of oxygen, especially by evaporating metal in the chamber which is incompletely oxidized as it is deposited. The second dielectric layer is then deposited to form a layer having a predetermined thickness. Additional dielectric layers may be formed as noted below.
In a preferred embodiment of the method coatings are formed on an optical element including adhesion and dielectric coatings formed by means of a process of physical vapor deposition carried out using conventional low-cost equipment and technologies usually employed in the manufacture of the two-layered mirror coatings. Advantageously, the adhesion layer and the first and second dielectric layers are formed by evaporating only two materials. In a preferred embodiment these materials are Cr and Si.
In this way it has been shown possible to attain unexpected, improved adhesion properties at a cost comparable to that attained by the methods for forming two-layered mirror coatings. In fact, even using the low-cost coating equipment and technologies usually employed in the manufacture of the two-layered superficial mirror coatings, the method of the present invention advantageously allows the production of improved mirror coated articles of a quality comparable to that of multi-layered coatings which are much more expensive and produced with sophisticated techniques, such as those based on the use of electronic guns. Moreover, this preferred manufacturing method of the present invention allows increase in the productivity with respect to the methods of the prior art employed for the manufacture of multi-layer coatings thanks to the reduced process time of this less sophisticated equipment.
The foregoing has been provided as a convenient summary of aspects of the present invention. However, the invention sought to be protected is defined by the claims.